Paul Aveyard, a professor at Oxford University, said: ‘It gets round the problem that people don’t think their child is overweight’

“They tend to use visual assessments – they look at the children and compare them to others in the school. So unless a child is really large they’re deemed to be fine.” She said showing parents the images had a significant impact on their children’s weight a year later.

Dr Jones added: “It’s tapping into parental concern about their children’s health in the future. We thought this would be a useful tool to pop in and parents really liked it.” The team are hoping to make the tool available on the NHS Choices website for families and health professionals to use.

The research was presented at the European Congress on Obesity in Porto.

Lead researcher Dr Angela Jones said: ‘We know parents tend not to recognise when their children is overweight or obese’

Swallowing an obesity 'BALLOON could help you shed two stone'

It looked at 2,210 children in the first year of primary school from 15 different parts of England. A total of 334 kids were overweight or obese.

Experts found 12 months on, 68 per cent of fat youngsters given the intervention had shed some of their excess weight.

Paul Aveyard, professor of Behavioural Medicine at Oxford University, said: “It gets round the problem that people don’t think their child is overweight and they will grow out of it, when we know that’s not really true.

“It’s quite a striking result and it’s cost free.” And Oxford Professor Susan Jebb, the government’s former obesity tsar, said: “If you can show what your child looks like as an adult you can think ‘this is the kind of person who looks like they’re going to have a heart attack’.”